The Doppler effect

Sound from the siren of a vehicle rushing to the scene of an emergency may appear to have a higher frequency as the vehicle approaches the scene and a lower frequency as it leaves the scene. This apparent change in the frequency of sound, which can be due to motion of the source or the observer, is called the Doppler effect.

The Doppler effect is named after the Austrian physicist Christian Johann Doppler (1803-1853), who developed formulae to express the change in frequency as a function of the change in velocity. These were verified, only three years later, in 1845 by a Dutch meteorologist, Christoph Heinrich Dietrich, who used trumpets as the source of sound and trains to transport the trumpeters and observers, in turn.

Fans of Indy car or Formula One racing may be familiar with apparent changes in frequency as the cars speed past them. If the cars are approaching the spectators, they hear an increase in frequency, and as the cars speed away the frequency appears to decrease.